1. Field of the Invention
A certain aspect of the present invention relates to an image scanning device for scanning a document to obtain an image of the document and a copier for forming the image obtained by the image scanning device.
2. Description of the Related Art
In a general document scanning process performed by a scanner, a target surface of a document placed on a contact glass is illuminated through the contact glass with light emitted from a light source on a carriage of the scanner while moving the carriage along the target surface. The light reflected from the target surface of the document is guided into an image sensor. Thus, an image of the target surface is read in a sub-scanning direction, i.e., in a direction the carriage is moved.
Some scanners are capable of automatically determining the size of a document placed on a contact glass. For example, patent document 1 discloses a scanner capable of automatically determining the size of a document. The disclosed scanner includes a platen cover used as a pressing unit for pressing a document against a contact glass. The platen cover is configured to swing open and close around a hinge. When closed, the platen cover covers the entire area of the contact glass and presses a document placed on the contact glass. When opened, the platen cover is fully retracted from the space right above the contact glass into a substantially upright position. When an opening operation of the platen cover is detected by an opening/closing sensor, a carriage of the scanner is caused to move from a home position (standby position) at one end of a moving range of the carriage to a document-size detection start position. The document-size detection start position is set at a distance of several cm from the home position. Meanwhile, when a closing operation of the platen cover is detected by the opening/closing sensor, a document on the contact glass is scanned with an image sensor by moving the carriage from the document-size detection start position toward the home position to obtain an image of the document. The size in a main-scanning direction (a direction orthogonal to the direction of carriage movement) of the document is determined based on the obtained image. Also, the size in the sub-scanning direction of the document is determined based on a detection result from a document detection sensor used to detect the presence of a document on the contact glass at certain positions along the sub-scanning direction. Thus, the disclosed scanner can automatically determine the size of a document and therefore eliminates the need for the user to enter the size of the document.
[Patent document 1] Japanese Patent Application Publication No. 11-196235
However, with the disclosed scanner, certain operations of the platen cover by the user may cause a problem called “chattering” where the moving direction of the carriage is abruptly reversed. Take, for example, a case where a user notices that he/she has forgotten to bring a document while opening the platen cover, and then immediately closes the platen cover. Here, if the platen cover is closed before the carriage, which has been caused by the opening operation of the platen cover to start moving from the home position toward the document-size detection start position, reaches the document-size detection start position, the carriage abruptly starts moving in the reverse direction. Thus, when the platen cover is opened and closed within a short period of time, the moving direction of the carriage is abruptly reversed, i.e., chattering occurs. The chattering greatly stresses and may damage the carriage and its drive mechanism.